Treatment of anthranilic acid in an anaerobic expanded granular sludge bed reactor at low concentrations

The mineralization of anthranilic acid (2AB) as the only carbon and energy source was studied in batch and continuous conditions using methanogenic granular sludge. Under batch conditions in serum vials, 2AB (300 mg/l) was completely mineralized to methane within 55 days time. The experiment with the anaerobic continuous expanded granular sludge bed (EGSB) reactor was initially conducted at an upflow velocity (V(up)) of 5 m/h, a hydraulic retention time (HRT) of 12 h and an organic loading rate (OLR) of 1.5 g chemical oxygen demand (COD)/l-d. After 102 days, 2AB was not degraded at all and the reactor operation was shifted to batch mode by recycling the effluent. After some days, 2AB was completely mineralized and accumulation of flocculent sludge was observed. Batch biodegradability assays demonstrated that this flocculent biomass had a higher specific biodegradation rate compared to the granular sludge. During EGSB reactor operation, the flocculent biomass which was not attached to the granules probably washed-out under the applied hydrodynamic conditions. When the EGSB reactor was operated at V(up) of 2 m/h to favour the retention of the flocculent biomass, 2AB was mineralized even at influent concentrations as low as 140 mg COD/L. Kinetic calculations indicated that the sludge had an apparent K(s) value for the mineralization of 2AB as low as 24 mg COD/l.